I started running into slow renders when I bought my Canon XF300s a year and half ago, I shoot at the highest setting, 50 mbs and immediately noticed slow renders, especially with any correction. An hour ceremony was taking me 10 hours to render almost any HD format. Prior to my canons, I had Sony V1us, which were not as intensive on the machine.
As for the mac, I had planned to phase out of Vegas and go FCPX full on, then I ran into a pretty nasty metadata problem and have been talking with Apple for the past month trying to work around the ‘camera name’ glitch in fcpx’s multicam.
Which leads me to a decision, rebuilding my pc since Vegas doesn’t have issues with my MXF files, except for the really slow renders.